You can use Final Gather to produce image-based lighting (or reflection) with a High Dynamic Range Image (HDRI). Image-based lighting takes the light (and light color) represented in an image you provide to illuminate the scene. An HDRI image has an extra floating point value associated with each pixel that is used to define the persistence of light at that point.
A high-dynamic range image is like several images with different exposures combined to show the full range of light (highlight and shadow). In fact, some HDR images are created by compositing several standard images of varying (bracketed) exposure in a special HDR application. This is required to simulate the wide range of available light in a single image -- an HDR image.
HDR images have a greater capacity to describe light accurately (by way of floating point numbers) because they store the amount of light (rather than just color) represented in a pixel. This prevents 'blown out' or extremely dark areas in an image that your eyes compensate for in the natural world.
Using HDR images with Final Gather lets you provide extremely realistic lighting.
To use HDRI images as sources of light and reflection, see Image-based lighting (sky-like illumination).
